Optical measuring process and precision measuring machine for determining the deviations from ideal shape of technically polished surfaces

ABSTRACT

In a known measuring process, the stability for the generation of two measuring beams, by means of two measuring systems operating in the same measurement strategy is given. The reflection angle of the measuring beams deflected onto a blank are detected as inclination deviations of the surface and analyzed by difference. Systematic measuring deviations are conventionally reduced however mainly by the use of a single measuring strategy or system with a moving measuring beam. According to the invention, the measurement accuracy can be improved by combining two measuring strategies in the measuring process, carried out by different measuring systems ( 13, 14 ), which can be an autocollimator (AKF) and a long-trace profilometer)LTP), the measuring beams of which can be directed at the blank ( 10 ) using different types of deflecting units ( 15 ). Measured results with an accuracy of up to 0.01 angle seconds, hence sub-nanometer range, for example +/−0.2 nm, can be achieve by means of suitable correlation of the measured values obtained from the different scanning methods for offsetting the systematic measuring deviations of both measuring systems ( 13, 14 ). Precisely produced surfaces of almost any dimensions, for example, nanometer optical components can thus be highly precisely inspected.

The invention relates to an optical measuring method of determiningdeviations from the ideal of technically polished surfaces with a planaror curved contour of a slidably mounted specimen by reference-freeoptical scanning with at least two measuring beams each of which isdeflected to the surface of the specimen by a slidable beam deflectionunit detected by the respective angle of reflection, and to a precisionmeasuring apparatus for carrying out the measuring method.

The requirements placed on a measuring method of this kind and on theprecision measuring apparatus carrying it out are extremely high. Ingeneral, the technical surfaces to be measured have a constant contourand may be planar or arbitrarily curved. However, the surfaces may alsobe provided with cracks and displacements. With such characteristics,the technical surfaces in their generally multifarious shapes constitutean essential feature of technical components. The extraordinaryspeciality of their features relates to the generality of the possibleshapes, the dimensions of the technical components and the precisemaintenance of precision of the shape. Fields of application to bementioned of such technical components may be, for instance, theautomotive industry, the turbine industry or optics. Possibleapplications of high precision measuring methods and precision measuringapparatus for practicing the method are individual and repetitivemanufacture, identification at manufacturers and users as well as use inresearch and development. In the automotive industry, during the courseof technical development, crank shafts, cylinders or pistons haveincreasingly complex functional surfaces deviating in their desiredshape or as a result of wear or under differing temperature conditionsfrom exact mathematical cylindrical shapes. Although such deviations maybe very small, yet the requirements regarding to their precision asdictated by their intended use set limits in respect of measurementuncertainties of up the 10 nm. In the turbine industry, the shaping ofthe structure of turbine vanes, fans or ships' propellers is optimizedat a very high differentiation by methods of finite elementcalculations. The surfaces shapes of aspheric free forms required inconnection with such structures may be fabricated by extremely precisenumerically controlled machine tools so that the determination of suchdeviations of shape also requires measurement uncertainties notexceeding 10 nm. Inherent in optical manufacture and testing is therequirement for a highly precise definition of the shape of lightconducting and converting components such as, for examples, mirrors,lenses, optical grids. The traditional form measuring accuracy of suchcomponents in the order of 50 nm, nowadays is in many cases required tobe 5 nm. Furthermore, currently and in future aspheric rotationallysymmetric or non rotationally symmetric surface shapes which are subjectto high demands of precision, are gaining increasing importance.Moreover, there is a need for extreme extensions of optical components.For use in research with synchrotron radiation aspheric mirrors ofrectangular configuration are necessary which exceed 1,000 mm in lengthand which are of the greatest possible accuracy of form. The applicationof reflecting and dispersing components for extremely ultraviolet lightof wavelengths in the order 0.1 nm to 10 nm for use in photonlithography calls for aspheric surfaces of a precision under 1 nm. As aresult of the required accuracies of the shapes of such components theirsurfaces often are of such micro roughness that they reflect light, i.e.that they are quasi polished. The reflectance of these surfaces dependsupon the material of which the components are made and upon theircoating.

In respect of the reflectance of the surfaces to be measured, the fieldof application of the present invention spans the range of wave lengthsof visible light between 100% as with metals, such as, for example,steel or gold, and 4% as in the case of optical glass, or even lower.The applications of contactless measuring methods furthermore extend tovery sensitively coated components with layer thicknesses of less than100 nm which could easily be damaged by contact measuring methods.

Measuring apparatus involving the transformation of mechanical oroptically contacting measuring methods are known in the prior art; theyeither compare the entire surface to be measured simultaneously againsta reference surface (interferometric measuring methods) or they rasterthe surface point by point (deflectometric measuring methods). Measuringapparatus involving mechanical contact are known as well. Among these isthe coordinate measuring apparatus known from German patentspecification DD 218,667 B1 which for eliminating deviations ofmeasurements is provided with a bilaterally effective mechanical sensingsystem for carrying out localized difference measurements on a specimenagainst a real or virtual reference forming reference element. Theoptical interferometric measuring methods require reference surfacesshaped like the surface to be measured which for differently curved, forexample aspherically curved surfaces require a plurality of differentconfigurations of the reference surface. However, the reference surfaceslimit the accuracy of the results of the measurements. The claimedmeasuring method is to be considered to be an optically contactingdeflectometric measuring methods. The simple rastering of surfacesaccording to the deflectometric method also includes a number ofsystematic and accidental deviations of measurement (measurement error).

German patent specification DE 197 20 122 C2 discloses an opticalcontact-free method for determining an optical measuring value and isbased upon establishing the difference between two constantly spacedmeasuring values. In principle, this shearing method is based upon thedetection of a difference between two measuring values using a singleoptical measuring method. Based upon this, a measuring method requiringneither a reference nor calibration and a measuring system are knownfrom German patent specification DE 198 33 269 C1 for the high-precisiondetermination, reaching into the range of several nm, of the topographyof a surface which is at least approximately planar, in which twoconsecutive measurements are taken by a single measuring system formeasuring an angle difference. By applying the basic principle ofdifference measurement, systematic measurement deviations which would beadditively introduced into the measurements are eliminated. It is truethat the mentioned publication also teaches the use of two beamgenerating measuring apparatus for generating measuring beams with ashearing distance for carrying out such difference measurements. This isexpressly described, however, as being disadvantageous since theresulting deviations cannot be compensated. Thus, the mentionedpublication suggests angle measuring including a measuring strategyconverted by an autocollimator in which a pentagonal prism is used as abeam deflector. Such a prism is substantially invariant with respect toinclinations of its reflective surfaces, but it has manufacturingtolerances which may result in systematic measurement deviations. Since,in addition, deviations may occur about the axis in the scanningdirection during measurement as a result of an incorrect alignmentrelative to the angle, the publication proposes an arrangement in whichin addition to the measuring autocollimator two more autocollimators areprovided which are arranged in alignment with the measuringautocollimator and in a plane at a right angle relative thereto. At thispoint it is to be expressly mentioned, however, that rather thanreceiving measuring values, the two autocollimators serve only tocorrect the angle of the pentagonal prism. Measuring the difference iscarried out by one and the same autocollimator by shifting the measuringhead. The actual aim of the mentioned publication is to avoid the use ofseveral measuring systems for determining difference values.

A collimator makes it possible in a small space to present largedistances of measuring marks. Together with a telescope set at infinity,it can be used to determine differences of direction. In the basicmeasuring strategy, the lens of a telescope projects a image of a markin the focal plane. If a telescope mark, for instance an angle scale, isprovided in the focal plane, the difference in directions between thecollimator and telescope axes can be determined from the shiftingbetween the marks. Parallel shifting of the two axes does not affect theangle measurement. If the collimator and the telescope are arranged inparallel closely together, and a planar reflector is used for beamdeflection, as well as a common lens, the result is a so-calledautocollimator with geometric beam splitting. Tilting of the planardeflector from a position at a right angle relative to the axis of thelens is perceptively indicated. Compared to a single collimator, thesensitivity is doubled. In an autocollimator, the illuminated marks haveto be brought into tune with each other. Among others, combinations ofmarks are suitable for adjustment and focusing. In an autocollimatorwith physical beam splitting the telescope and collimator axes coincideup to a beam splitter. This relates to the standard instrument used formost measuring applications of the kind described.

A measuring method and an arrangement for measuring differences with oneand the same autocollimator are also known from German patentspecification DE 198 42 190 C1. In this case, too, the stated object isthe avoidance of using several measuring systems for determiningdifference values. This publication, upon the instant invention is basedas the closest prior art, relates to the same problem but, in this case,to determining the topography of perceptively curved surfaces. Here,too, an autocollimator is used in a measuring arrangement which takes upmeasuring values along a scanning line for establishing differences. Forimproving the precision, the scanning beam is always reset verticallyrelative to the scanning surface. In this arrangement, too, two furtherautocollimators may be used to correct the angle of the pentagonal prismused as a deflection unit. These also are not used to ascertainmeasuring values directly; but they serve as zero indicators and maynot, therefore, be considered to be measuring systems. However, thepublication also mentions an arrangement for establishing differencesincluding two autocollimators for generating two separate measuringbeams and to beam deflector units for measuring one angle position each.Even though this arrangement uses two autocollimators it neverthelessapplies only one measuring strategy. Moreover, the publication expresslypoints out that because of the unavoidable deviations of the synchronousguidance of the two measuring heads and deflector units the order ofmagnitude of the basic deviations is such that the attainable precisionis subject to considerable limitations. Thus, this publication alsogives preference to the use of a single measuring system with shiftingof the measuring beam between the difference forming measuring points bya deflector unit, since the additive systematic deviations thus enterinto both angle measurements and, therefore, are excluded in case of theestablishment of a difference. The publication discloses, furthermore,that by using a diaphragm with two apertures in the autocollimator, thetwo measuring beams required for measuring a difference may be formed bya single beam. It is thus known that an autocollimator cannot only emitone measuring beam but also, at least, two.

The mentioned actions notwithstanding, it is not possible, because ofoccurring systematic deviations, with the known measuring methods andarrangements to maintain measuring accuracies of the kind which will beincreasingly required in novel applications, particularly for opticalcomponents with dimensions between 30 mm and 1,200 mm and sharpercurves. In this connection, the technical goal to be mentioned is aprecision of measuring deviations from an ideal shape of angles ofinclinations of 0.01 seconds of arc, which in practice cannot currentlybe attained, however. The attainable 0.08 seconds of arc beinginsufficient, the object of present the invention is seen in attaining amean measuring precision of less than 0.01 seconds of arc rms (less than50 nrad rms) when detecting the mean deviations of inclination oftechnically polished surfaces. In this context, the variety of theshapes of specimens and components to be measured and the highsensitivity of the surfaces to be measured are to be taken intoconsideration. Furthermore, a short and automatic measuring process anda flexible use of apparatus is to be possible. The accomplishment of theinvention may be gleaned from the main method claim and from theassociated systems claim. Advantageous embodiments of the invention maybe gleans from the corresponding subclaims. They will hereafter beexplained in greater detail in connection with the description of theinvention.

The essential concept of the measuring method in accordance with theinvention and of the precision measuring apparatus for executing themethod is the combination of two or more different measuring strategiesor optical measuring systems in a hybrid arrangement. An overview of theprior art has shown that this constitutes an unusual measure which inthe invention yields a surprisingly great success. By contrast with theknown teaching to avoid, if possible, several measuring strategies orsystems, with the prior art teaching only the use of similar measuringsystems, the invention teaches the deliberate combination of two or moremeasuring strategies using specific but known measuring and evaluationstrategies and, correspondingly, of a plurality of measuring systemsdiffering, if possible, in their occurring systematic deviations ofmeasurement. Preferably, this may be a combination of the collimationmeasuring strategy and of the surface profile measuring strategy. Bymeans of an autocollimator preferred for executing the correspondingmeasuring strategy and a long trace profilometer (LTP) identicalmeasuring sites, or chronologically and/or spatially offset measuringsites, are simultaneously scanned in order to yield the required highlyprecise measurement result from the differentiated observation of theobtained measurement results. This makes use of the fact that the LTPalways provides two measuring beams (with, in one embodiment of theinvention, the reference beam in the surface profile measuring strategyor LTP being preferably used as a measuring beam by deflecting thereference beam to the specimen), the autocollimator usually provides onemeasuring beam. However, the prior art also discloses arrangements whichuse at least two measuring beams (see German patent specification DE 19842 190 C1). Such an autocollimator with two or even more measuringbeams, which always enclose a set angle, may advantageously be used inthe context of the invention for increasing the number of the differingmeasuring sites for this measuring strategy. A further differentiationof the association of individual measuring beams with the givenmeasuring strategy or given measuring system may take place inaccordance with another embodiment of the invention, by using measuringbeams of differing wavelengths which are detected by appropriatefiltering and are assigned to appropriate measuring strategy. Thus,detected measuring beams at one and the same measuring site and at thesame measurement point in time may in a simple manner be neverthelessassigned to the different measuring systems. In principle, the measuringmethod in accordance with the invention allows the use differentmeasuring strategies of different resolution. However, where the usedmeasuring strategies are to ensure the same resolution, it isadvantageous in accordance with an embodiment of the invention togenerate substantially equally sized scanning sites by the usedmeasuring beams. Measuring beams of equal diameter are then generated bythe use of identical apertures.

The scanning of chronologically and/or spatially offset measuring sitesby different measuring beams of approximately equal beam diameter isbased upon the recognition that during a measuring operation or as partof the result of each measurement it is necessary constantly to analyzethe manifold influences by the measurement deviations which aredistinguished according to accidental and systematic errors. Theaccidental deviations must be constantly detected, analyzed and, wherenecessary, incorporated as a control parameter into the process forcorrecting the conditions give rise to them, into the measuringoperation and into the result of the measurement. In accordance with afurther embodiment of the invention it is advantageous in acomputer-assisted control of the slidable elements to provide for acombined cooperation with the measuring values detected by the at leastto different measuring strategies, in special measuring and adjustmentstrategies after semi or fully automatic measuring and specimenadjustment strategies and for storing the measuring and position valuesin a storage unit. To accomplish this, it is necessary to provide forthe conditions in terms of measuring techniques in order to minimize theaccidental components from the different measuring values. For thispurpose, the apparatus in accordance with the invention offers, in itsbasic concept of executing the combined measuring operation and in itspreferred advantageous embodiments, a number of effective measures.Among these are, in particular, the entire constructive design of theclaimed precision measuring apparatus and the avoidance or minimizationof environmental influences. This and the possibility of a rapidacceptance of large quantities of measuring data makes it possible toeasily to control the accidental and statistically occurring measurementdeviations, so that their effect upon the accuracy of the measurementscan be reliably eliminated. It is completely different in the case ofsystematic measurement deviations which must be categorized according totheir possible causes. However, the combination of two differentmeasuring strategies renders them clearly recognizable, and by applyinggenerally known evaluation strategies, such as, for instance, thedifference formation, they can be compensated or eliminated, as the casemay be. In this connection it is to be noted, that especially higherorder measurement deviations also, which must be taken intoconsideration in such precisely to be determined measuring values, atsimilar environmental conditions for both measuring systems are alsodetected by the measuring method in accordance with the invention.

The long trace profilometer (LTP) may be an optical measuring apparatusfor checking the surface formation of slightly or uncurved opticalsurfaces of large extent and highest precision, as known from the basicpatent, viz.: U.S. Pat. No. 4,884,697. The LTP operates on the basis ofthe surface profile measuring strategy, with a double beam (themeasuring and reference beam) the reflections of which from the specimenare detected at the site of a line detector. An interference image isgenerated at the site of the sensor the site of the image on thedetector being a measure of the inclination of the specimen at any givenscanned site. The inclinations of a specimen along a straight measuringpath are measured directly by the LTP which, like an autocollimator, maybe driven by a laser source. This makes apparent the basic differencefrom the measuring strategy of an autocollimator. The autocollimator canalways measure and correct two values of angles disposed at right anglerelative to each other, whereas the LTP in its general applicationmeasures in the longitudinal direction of the specimen only, but at alarger angular measuring range. With spherically curved surfaces inparticular, the measuring range can be increased by the use of an LTP,and a greater extent of a specimen can be measured as well. By the smallapertures of the autocollimator which are adapted to the measuring beamsof the LTP and by the point measurements of the LTP an especially highspatial resolution can be achieved by the measuring method or with theprecision measuring apparatus in accordance with the invention, as aresult of the small diameters which is substantially identical for allthe measuring beams. The high spatial resolution in turn ensures thequantity of measuring values important for the elimination of theaccidental measurement deviations and the detection of all surfaceinclinations even in the smallest ranges.

The highly precise measuring method and the precision measuringapparatus in accordance with the invention which because of itsattainable measuring accuracy may be called “ultra precision measuringapparatus” make possible applications in a wide field as a result of thebasic combination of two different measuring strategies or systems and anumber of additional advantageous embodiments. Thus, the large number ofthe basic shapes of the specimen surfaces to be measured may extend overa very wide range of possible constant surface formations. For instance,it is possible to measure the inclination of deviations from a plane canbe measured of surfaces, cylindrical surfaces, spherical surfaces,rotationally symmetric aspherical surfaces, non-rotationally symmetricaspheric surfaces, surfaces of the shapes of basic conical sections sucha ellipsoids, toroids, paraboloids or elliptical cylinders and evenaspherical free forms or specimens with interrupted surfaces or surfacesbent along a sharp separation line. In general, the constantly extendingcurvature of the surfaces to be measured is to be subject to smalllocalized changes in curvature. However, more pronounced curvatures maybe measured as well by the use of special measuring strategies which areknown per se. The variety of the component formations to be measured isnot limited to circular round parts with a cylindrical edge, but it alsoincludes rectangular shapes or shapes delimited by round surfaces eventhose of extreme length to width ratios or length to thickness. Thesensitivity of the surfaces to be measured in respect of the risk ofdamage, especially in the case of coated surfaces, and in respect ofcleanliness to maintain the quality for use in ultra high vacuum istaken into consideration by the two strictly optically scanningmeasuring systems. The optical degree of reflection differing from zeroof the measured surfaces may, for instance, be between 4% and 100%. Inthe case of highly sensitive detectors it may even be below 4%. Shortterm measurements conforming to the necessary measuring accuracy may beexecuted without any problems, especially by the automation, in a mannerparticularly suitable for industrial repetitive manufacture, of the twoused measuring systems in respect of the measuring operation and theexchange, adjustment and guidance of the specimen to be measured. Themeasuring speed may easily be adapted to the required accuracy.

The use of two different optical measuring strategies or measuringsystems ensure the appearance of different systematic measurementdeviations. For that reason the measuring values derived from twomeasuring systems at two scanning sites need only be processed to acommon measurement result by suitable evaluation strategies in orderreliably to eliminate any systematic errors. The appearance of differingsystematic errors is, therefore, of the utmost importance. Equalsystematic measurement deviations which would occur by using twoidentical measuring strategies or systems cannot yield the accuracy ofmeasurement extending into the required range of accuracy. The inventionthus demonstrates that it is exactly the improvement of the concept,avoided by the prior art on grounds of being disadvantageous, of usingtwo different measuring strategies or measuring systems in a hybridarrangement which yields results. The extremely accurate measuringresults attainable by the measuring strategy or precision measuringapparatus according to the invention which will be described in thespecific section of the specification, are confirming this concept. Forcorrelation in the evaluation unit, suitable yet well known evaluationstrategies of autocalibration and reduction of accidental and systematicmeasurement deviations from the different measurement deviations fromthe autocollimation telescope and the long trace profilometer have to beselected and balanced against each other. Different processes are knownfor autocalibrating the two measuring systems. The specimen may, forinstance, be scanned repeatedly. Between scanning cycles the specimenmay be rotated by 90° or 180°. Moreover, different scanning paths may beoperated in parallel. Moreover, the angles of inclination in particularmay be measured at the same or different positions of the specimen atthe same or different points in time. The reception of two measuringvalues at one measuring site at one measuring time will certainly yieldthe least measurement deviations, whereas a chronologically shiftedmeasurement at two different measuring sites which are sufficientlycorrelated for detecting a common measurement result, will yield thelargest measurement deviations. Finally, it is possible in a suitablemanner to combine the different species of the two measuring systems.For instance, the three measuring beams of the two measuring systems (ameasuring beam from the autocollimator, and two measuring beams from theLTP, where one measuring beam may be a reference beam) or just twomeasuring beams (one beam from each measuring system) may be drawn uponfor evaluation. In its simplest form, the evaluating correlation of thedetected measuring values may be carried out, for instance, by forming amean value. In that case the affects of the two different measuringsystems are included in the weighting at a 50% share each. Thedifference formation of the kind previously explained in greater detailin connection with the shearing interferometry is possible as well. Inthe last analysis, however, the correlating processing of severalmeasuring values to a common measurement result is a task which isgenerally familiar to, and solvable by, a skilled artisan on the basisof his experience. In this context, a computer-assisted evaluation cangreatly simplify the solution of the requisite algorithms.

A number of constructive measures regarding the optical and mechanicalrequirements improves the measuring accuracy of the claimed precisionmeasuring apparatus in accordance with the invention even further. Amongthese, for instance, is the aligned disposition of the optical axes ofthe two measuring systems, so that there is no need to take intoconsideration any misalignment in the deflection of the beam.Furthermore, the design of the reflecting deflection units is ofsignificance. Advantageously, the beam deflectors may consist of tworeflective planar mirror surfaces which are rigidly arranged relative toeach other with an orientation of the intersecting edges at a rightangle relative to the shifting direction of a measuring translationslide. This results in considerable invariance as regards tilting of thereflecting planes. It is only the occurring deviation of inclination atthe polished surface of the specimen which is to be measured. The beamdeflection units may also consist of double mirrors the reflectingsurfaces of which are disposed at an angle of δ=45° relative to eachother and the normal of the surface of which is disposed at δ/2 eachrelative to the emitted measuring beams of the two measuring systems andto the measuring beams reflected from the surface of the specimen. Inthis manner any effects of inhomogeneity from the optical glass of thepentagonal prism are avoided. Optionally, the reflective surface mayalso be disposed at an angle (±α, ±β) different from δ=45°, so thattheir surface normal is disposed at a difference of half thecorresponding angle to the measuring system and to the measuring beamreflected from the specimen. Both geometric arrangements may simply beincluded in the evaluation strategy. Finally, it is of particularadvantage to structure the deflection units as pentagonal prisms. Theirinvariance relative to tilting is generally known. Thus, there is noneed in the precision measuring apparatus of the invention foradditional optical arrangements to provide for highly preciseadjustments of the pentagonal prism.

Environmental influences exert particular affects upon the accidentalmeasurement deviations and, hence, upon the attainable measuringaccuracy as well as upon the measuring method or precision measuringapparatus. Even though accidental measurement deviations can besubstantially eliminated by many measuring values, it is advantageoussubstantially to avoid accidental measurement deviations. Among relevantmeasures is the protection of free light paths by longitudinallyvariable and steady air separation devices. In this manner interferingair turbulence around and within the light beams can be avoided.Furthermore, the structural arrangement of the apparatus over differentslides relative to the support and movement of the specimen and themovement of the optical deflector units is significant. For instance,the measuring translation slide may be moved by a separate drive slide,and the two slides may be connected by a coupling which is elasticallystructured relative to the rotational shifting directions. In thismanner interferences, for instance vibrations originating with the slidedrive system, can be compensated by the elastic coupling. The measuringaccuracy may be further improved by an orthogonal and parallel alignmentof the slidable elements relative to each other and to the measuringbeams, for instance by arranging the positioning slide at a right anglerelative to the measuring translation slide the table surface of whichis aligned parallel to the shifting surface and at a right angle to thedirection of the measuring beams. The same holds true for an arrangementin which a carriage is arranged on the table surface of the positioningslide, with the guidance direction of the carriage being disposedparallel to the measuring translation slide. A further influentialfactor is the support and shifting of the specimen. For this purpose, itis advantageous to provide a precision turntable on the carriage therotational axis of which is disposed orthogonally relative to theshifting directions of the measuring translation slide as well as of thecarriage. In addition, the precision turntable may have a pivotalrotational axis for additionally executing a pivoting movement. Forsupporting the specimen on the precision turntable, the latter may beprovided with a receiving and adjustment device for the specimenconsisting of a receiving table having three support studs disposed in arectangular, isosceles or equilateral triangle and of which at least twoare vertically adjustable and the planar table surface of which isdisposed parallel to the table surface of the lateral slide and which bythe vertically adjustable studs may be tilted about the rectangularlydisposed shifting axes of the two slides.

With a view further to improving the quality of the precision measuringapparatus in accordance with the invention, it is advantageous tocontrol its set positions. For this purpose, the slidable elements maybe equipped with measuring sensors for determining a given position. Asregards setting of a precise position, it is of further advantage toprovide electrical drive systems for the slidable elements. Moreover,during a measuring operation the sliding movement of the slidableelements may in cooperation with the issuance of measuring values by thetwo measuring systems be advantageously combined with the aid of acomputer in special measuring and adjustment strategies by semi- orfully automatic measuring and specimen adjustment strategies, and themeasuring and positional values may be stored in a storage unit. It isalso advantageous to avoid any heat-emitting elements, or to minimizetheir heat dissipation, in order to keep thermal effects as low aspossible. To this end all the drive and measuring systems may beoperated at a very low electrical dissipation and only one drive systemand the measuring systems may be operated during the measuringoperations. An embodiment in which the various bearing arrangements areconstituted by air bearings is of very low loss and free of friction. Inthe same manner as the positions and dispositions of specimens and allslidable elements can be detected by sensors, secondary sensors mayadditionally be provided for sensitive apparatus groups for determiningdifferences of temperature, vibrations, relative humidity andatmospheric pressure, the measuring values of which will be included inthe evaluation. Finally, in order to insulate it from the environmentand, hence, detrimental environmental effects, the entire precisionmeasuring apparatus may enclosed in a temperature insulating andsubstantially airtight housing.

For a better understanding, embodiment of the invention will hereafterbe described in greater detail with reference to the schematic drawings,in which:

FIG. 1 depicts the basic structure of the precision measuring apparatusin accordance with the invention;

FIGS. 1-10 depict different scanning variants of the two measuringsystems;

FIG. 11 a represents a detected surface relief of a planar gridsubstrate;

FIG. 11 b represents a center scan of the grid substrate of FIG. 11 a;

FIG. 11 c depicts the reproducibility of the measuring results on thegrid substrate of FIG. 11 a;

FIG. 12 shows the measuring result from an elliptical cylinder;

FIG. 13 depicts a comparison of measuring results; and

FIG. 14 depicts an elevational profile.

FIG. 1 depicts a precision measuring apparatus 1 in accordance with theinvention of an attainable measuring precision into the sub nanometerrange (0.01 seconds of arc rms or, correspondingly, 0.03 nm mean rms).It consists of a stone base 2 (for instance granite), upon which a stonecross beam 3 is mounted. A position slide 4 can be moved on an aircushion over the depth (y direction) of the stone base 1, rectangularlywith respect to a measurement translation slide 5. The measurementtransmission slide 5 is supported by the cross beam 3 and may be movedover the free width of the stone cross beam 5 (x direction). To move theslide 5, a drive slide 6 is provided which also moves on the stone crossbeam 3. A carriage 7 is mounted on the position slide 4 at a right anglerelative to the measurement translation slide 5, the table surface ofthe carriage 7 being disposed in parallel relative to the plane ofmovement and vertically relative to the deflecting measuring beams (seeinfra). On the carriage 7 there is mounted a precision turret 8 which isprovided with pivotable rotational axis for executing pivotal and rotarymovements by means of a pair of bearings. In the selected embodiment, abeam-like specimen 10 (P) is mounted on the precision turret 8 by meansof a multiply adjustable reception and adjustment device 9.

At the sides of the precision measuring apparatus 1, two measuringsystems 13, 14 are placed opposite and facing each other with theiroptical axes in alignment, on stone supports 11 placed upon the stonebasis 2. The right measuring system 13 is a autocollimation telescopeAKF; the left measuring system 14 is a long trace profilometer LTP theoperating strategy of which, as is well known, operates by a measuringstrategy completely different from that of the AKF. By suitablycorrelating, in an evaluation unit not shown in the drawing, themeasurement values obtained by the two measuring systems 13, 14,systematic measurement deviations can be minimized and highly accuratemeasurement results can be obtained, as has already been set forth ingreater detail in the general description. In both measuring systems,the measuring beams are directed against beam deflection units 15 (M)which are connected to the slidable measurement translation slide 5 andwhich serve top deflect the beams onto the specimen 10. The beamdeflection unit 15 may consist of two mirrors aligned at a predeterminedangle relative to each other, or, especially, a pentagonal prism, andwhich are especially constant against insignificant tilting of thereflectors. Different possible embodiments of the beam deflection unit15 and of the utilization of the measuring beam are described inconnection with the following figures. The simplest case of a commonbeam deflection unit 15 for all measuring beams is being mentioned herefor reasons of completeness. FIGS. 2 to 10 depict different possiblestructures of beam deflection units M, the utilization of the measuringbeams of the two measuring systems AKF addn LTP and of the scanningvariations at one or at several measuring points. As I well known, theAKF usually emits one measuring beam, whereas the LTP emits twomeasuring beams, where one of the measuring beams may be constituted bythe reference beam inherent in the LTP. Each of the detection planes ofthe AKF (two planes) and of the LTP (usually one plane) are shownschematically. Both measuring systems require a light source forgenerating the measuring beams. Preferably, this is laser light source.It may be integrated in the AKF and is, therefore, not shown in thedrawings. The LTP is provided with a more powerful laser which becauseof its heat generation is not integrated into the LTP proper; rather, itis mounted externally. This condition has been indicated in the drawingsby the light source Q. Behind the light source Q there is provided abeam splitter D for generating the two measuring beams of the LTP. Thecorresponding angles of the reflector relative to each other within thebeam deflection units M have been shown in the drawings. The 45° angle(δ) as the base angle depicts the vertical impingement of the light uponthe surface of the specimen. For changing this base angle to provideoblique impingement of light, the angles α, β have been shown for thetwo measuring systems AKF and LTP which have to be correspondingly in acounter-directed disposition (±α,±β). Furthermore, the directions of thearrows is to be noted in order to be able to distinguish the emittedmeasuring beams from the reflected ones. Measuring beams impingingorthogonally upon the surface of the specimen P may be fed back to thedetectors by a suitable modification of the beam deflection unit M. Theindicated scans may be synchronous as well as offset in time; or theymay be identical or offset in respect of site, or they may occur in anydesired combination of time and site. The selection takes placedependent upon actual ambient condition and other environmentalparameters (Process speed, number of measurements, evaluations, etc.).

FIG. 2 depicts a spatially identical scan by a measuring beam from theAKF and two measuring beams from the LTP, one of the two measuring,beams from the LTP being vertically directed upon the surface of thespecimen P. FIG. 3 depicts an identical construction, however, withoutthe vertical measuring beam. FIG. 4 also depicts a spatially identicalscan with an orthogonal measuring beam from the AKF and two measuringbeams from the LTP which are not orthogonally aligned. FIG. 5 shows asynchronous or chronologically offset scan at one site with anon-orthogonal measuring beam from the AKF and an orthogonal measuringbeam from the LTP. The following figures depict spatially offset scans(two or three measuring points). FIG. 6 depicts a scan with onemeasuring beam from each measuring system AKF, LTP, both measuring beamsbeing orthogonally deflected on to the surface of the specimen P. FIG. 7depicts a spatially offset scan with three measuring beams all of whichare orthogonally aligned. Finally, FIG. 8 depicts a spatially identicalscan of a measuring point by both measuring systems AKF and LTP and aspatially offset scan of a further measuring point which may take placechronologically offset. Both measuring beams from the LTP impinge thespecimen P orthogonally, whereas the measuring beam from the AKF isimpinging angularly. FIG. 9 shows a spatially offset scan of a specimenP by three measuring beams from the AKF at three different measuringpoints and a simultaneous scan of a further measuring point by the LTP.FIG. 10 shows a spatially identical scan of a specimen P by twomeasuring systems using light of different wave lengths λ₁ for onemeasuring system LTP and λ₂ for the other measuring system AKF.Detection takes place by appropriate filters.

FIGS. 11 to 14 depict results of measurements determined by the methodor precision measuring apparatus in accordance with the invention, asthe case may be. FIG. 11 a depicts a surface relief of a grid substrateof mono-crystalline silicon taken and correspondingly evaluated by anAKF and a LTP. The specimen P is of a length of 100 mm at a width of 20mm and height of 40 mm. The spatial resolution is 3 mm; 180 longitudinaltraces were taken at a spacing of 0.1 mm. Over a length of 90 mm (scanposition) the surface relief evinces a maximum deviation of about 8 nm.Hence, it is a highly planar grid substrate with a radius ofapproximately infinite curvature (100 to 200 km). The measurementuncertainty for the shape is smaller than the value of ±0.5 nm. FIG. 11b depicts a determined mean trace over an extent of 90 mm. The maximumpeak to valley height is 9.1 nm. The result is a deviation of curvatureof 2.7 nm rms, corresponding to 72 milliseconds of arc rms (root mainsquare). FIG. 11 c depicts the high reproducibility of the measurementcurves taken of the height deviations and proves the high accuracy ofthe ultra precision measuring apparatus in accordance with theinvention. The reproducibility as a difference of two results averagedfrom each of 6 measuring lines amount to from MW6 to MW8±0.24 nm peak tovalley at a deviation of curvature of 0.13 nm rms, corresponding to 11milliseconds of arc rms.

FIG. 12 depicts a three-dimensional height curve for an ellipticalcylinder made of glass ceramic and of which a measuring surface of 120mm length and 25 mm width was scanned. The spacing between measuringpoints was 1 mm in the x and y directions. FIG. 12 discloses thecylindrical shape of the specimen. The profile represents an ellipse themedian deflection of which is 97.103 μm at a median circular deviationof ±4.684 μm. The comparison of the result of the measurement with theellipto-cylindrical desired surface not shown here results in a mediandeviation of shape of the specimen of 120 nm rms.

FIG. 13 shows a comparison of the measurement result of the originallyobtained deviations of inclination (right) and the deviations of height(left) derived therefrom of a spherically curved highly precise mirrorused as the specimen by individually used measuring systems (LTP at thetop and AKF in the middle) and their combination in accordance with theinvention (at the bottom). The deviations are shown as deviations fromdesired coordinates of the median sphere. The lower line depicts thecombination of the two individual measurement curves by averaging andzero displacement. The curve thus represents the combined overallresult. The depicted results of the measurements for the deviations ofinclination and height indicate that the results expressed by thenumbers as maximum deviations (peak to valley) and as mean squaredeviation (standard deviation rms) in the combination are smaller in allcases than in the individual measurements. The reason for this residesin the different incorporation of the systematic measurement deviationsof the individual sensors into the overall result. The combined resultthus is more precise than the results from the individually operatedmeasuring systems.

FIG. 14 is a presentation of the height profile of the opticallyeffective surface of a specimen measuring 510 mm in length, 120 mm inwidth and 120 mm in height. The measured median peak to valley height is20 nm. The height lines are spaced 2 mm from each other. Thelongitudinal radius amounts to 500 km, the measured median deviation ofcurvature amounts to 0.06 milliseconds of arc rms, which in themeasurement by the precision measuring apparatus in accordance with theinvention is exactly within the attainable range of precision.

1. An optical measuring method for determining deviations from an idealform of technical polished surfaces of planar or curved extent of aslidably mounted specimen (10) by reference-free deflectometric scans byat least two measuring beams each of which is deflected to the surfaceof the specimen by slidable beam deflection units (15) and detected bythe corresponding angle of reflection, the measuring beams beinggenerated and detected by at least two different optical measuringstrategies and the scanning sites dependent on the total number ofscanning beams being selected as spatially and/or chronologicallycoincident and/or offset, and by a combining evaluation of themeasurement values detected at the scanning sites as a commonmeasurement value by application of selectable evaluation strategies ofautocalibration and reduction of accidental and systematic deviations ofmeasurement from the at least two different optical measuringstrategies.
 2. The optical measuring method of claim 1 with generationof scanning sites of substantially equal size by the measuring beamsused.
 3. The optical measuring method of claim 2 with at least twomeasuring beams or different wave lengths associated with the differentmeasuring strategies which are detected by corresponding filtering andassociated with the corresponding measuring strategy.
 4. The opticalmeasuring method of claim 3 with a combination of the collimationmeasuring strategy with the surface profile measuring strategy.
 5. Theoptical measuring method of claim 4 with generating, by the collimationmeasuring strategy, several measuring beams enclosing a constant anglebetween.
 6. The optical measuring method of claim 5 with a deflection ofthe reference beam in the surface profile measuring strategy asmeasuring beam onto the specimen.
 7. The optical measuring method ofclaim 6 with a computer-assisted control of the slidable elements incombined cooperation with the measuring values detected by the at leasttwo measuring strategies in special measuring and adjusting strategiesand storage of the measurement and position values in a storage unit. 8.A precision measuring apparatus (1) for practicing the highly precisemeasuring method of claims 1 to 7 with an arrangement of the specimen(10) on a positioning slide (4) slidable over the depth of theapparatus, with a connection of the beam deflection units with ameasurement translation slide (5) slidable over the width of theapparatus, with a measuring system (13, 14) including a detection unitfor each measuring strategy used, the at least two measuring systems(13, 14) being positioned opposite each other at a distance extendingover the width of the apparatus and arranged in parallel to each otherwith respect to their optical axes and the measurement translation slide(5), and with a control and evaluation unit for selecting the scanningsites and evaluating the detected measurement values.
 9. The precisionmeasuring apparatus of claim 8 with the at least two measuring systems(13, 14) arranged with their measuring axes in parallel with each other.10. The precision measuring apparatus of claim 9 with an autocollimationtelescope (AKF) for executing the collimation measuring strategy and along trace profilometer (LTP) for executing the surface profilemeasuring strategy.
 11. The precision measuring apparatus of claim 10with a structure of the beam deflection units (15) of two reflectingplanar mirror surfaces rigidly arranged with the intersection margins ata right angle relative to the sliding movement of the measurementtranslation slide (5).
 12. The precision measuring apparatus of claim 11with a structure of the beam deflection units (15) of double mirrors thereflective surfaces of which being disposed relative to each other at anangle of δ=45°±α, β and the surface normal of which being disposed atδ/2 relative to the emitted measuring beams of the measuring systems(13, 14) and relative to the measuring beams reflected from the surfaceof the specimen (10).
 13. The precision measuring apparatus of claim 12with a structure of the beam deflection unit (15) as a pentagonal prism.14. The precision measuring apparatus of claim 13 with a protection ofthe free light paths by longitudinally adjustable and rigid airseparation devices.
 15. The precision measuring apparatus of claim 14with sliding of the measurement translation slide (5) by a separatedrive slide (6) the connection between the two slides (5, 6) beingestablished by a coupling elastically mounted in the rotational slidingdevices.
 16. The precision measuring apparatus of claim 15 with arectangular arrangement of the position slide (4) relative to themeasurement translation slide (5), the table surface of the latter beingdisposed in parallel relative to the sliding surface and at a rightangle relative to the direction of the deflected measuring beams. 17.The precision measuring apparatus of claim 15 with an arrangement of acarriage (7) guided on the table surface of the position slide (4) inparallel relative to the measurement translation slide (5).
 18. Theprecision measuring apparatus of claim 17 with an attachment of aprecision turret (8) on the carriage (7) the rotational axis beingorthogonal relative to the sliding directions of the measurementtranslation slide (5) as well as the carriage (7).
 19. The precisionmeasuring apparatus of claim 18 with a structure of the is precisionturret (8) with a pivotable rotational axis for the additional executionof a pivoting movement.
 20. The precision measuring apparatus of claim19 with an arrangement of a receiving and adjusting device (9) for thespecimen (10) on the precision turret (8) consisting of a receivingtable having three supports disposed in a rectangular, isosceles orequilateral triangle at least two of which are vertically adjustable andthe planer table surface of which is disposed in parallel relative tothe table surface of the carriage (7) and which by the verticallyadjustable supports may be tilted about the rectangularly disposedsliding axes of the carriage (7) and measurement translation slide (5).21. (canceled)
 22. (canceled)
 23. (canceled)
 24. (canceled) 25.(canceled)
 26. (canceled)